Electromechanical signal translating apparatus



May 14, 19

L.. J. AAhlpnalasolw 2,406281 ELECTRO-MECHANICAL SIGNAL TRANSLATINGAPPARATUS v 2 Sheets-Sheet 1 Filed Oct. 31, 1940 :Summon lie-:ZlzirsamMay 14, 1946. l.. J. ANDERSON ELECTRO-MECHANICAL SIGNAL TRANSLATINGAPPARATUS Filed Oct. 51, 1940 2 Sheets-Sheet 2 Mar/v.; YJ, C

Snnentor (Ittorneg Patented May 14, 1946 ELECTROMECHANICAL SIGNALTRANSLAT- ING APPARATUS Leslie J. Anderson, Haddonfield, N. J., assignerto Radio Corporation of America, a corporation of Delaware ApplicationOctober 31, 1940, Serial No. 363,632

vs Claims.

invention to provide an improved signal translating device of this typewhich is particularly useful as a microphone for picking up andtranslating speech with good efliciency.

Another object of my present invention is to provide an improved signaltranslating device of the type set forth in which the mass of the movingparts is reduced to a minimum.

A further object of my present invention is to provide an improvedsignal translating device as aforesaid which, when used as a microphone,will not be appreciably subject to the disturbing effects of wind andlow frequency noises which may be present.

It is also an object of my present invention to provide an improvedreversible signal translating device as described above which isrelatively simple in'construction and highly efficient in use.

In accordance with this invention, I employ a sound powered telephoneunit and couple to the diaphragm thereof an acoustic network soproportioned that a substantially flat frequency response is obtainedover the desired range. The diaphragm is coupled through a, coupling ofsmall mass to a suitable reversible electro-mechanical unit, such as amagnetic motor or generator, as the case may bel the diaphragm beingmounted in a casing having a front plate and a back plate and beingspaced from said plates whereby an air space is provided both in frontof and behind the diaphragm. Both of the aforementioned plates areprovided with one or more openings which constitute acoustic pathsbetween said air spaces and the exterior of said casing, the openingsbeing preferably covered by silk or other suitable material constitutingan acoustical resistance, and one or more of the openings in the backplate behind the diaphragm may be covered by an enclosure providing a,small chamber. Coupled to the back plate of the diaphragm casing is ahousing which eneloses the reversible unit and the aforementionedenclosure, the openings in said back plate affording communicationbetween the air space behind the diaphragm and the interior of saidhousing. A tubular member extending into the housing from one of thewalls thereof affords communication between the air in the housing andthe atmosphere. lFor best results, I have found that the followinggeneral constants are desirable:

(a) The length and diameter of the tube which connects the atmospherewith the interior of the housing is preferably so chosen that the massof air therein resonates with the compliance of the air in the housingat the lowest frequency at which transmission is desired.

(b) The resonance of the mass of the complete moving system consistingof the diaphragm, the armature of the electro-magnetic unit, and thecoupling member therebetween with the compliance of this system isplaced as high in frequency as will allow the desired low frequencyresponse.

(c) The compliance. of the volume of air 'in the space behind thediaphragm, that is, between the back surface of the diaphragm and theback plate of its casing, is made small enough so that, in combinationwith the mass and compliance of the moving system, resonance will beobtained at the highest frequency desired.

(d) The compliance of the resonator volume, or the volume of air in thesmall chamber or chambers behind one or more of the openings in the backplate of the diaphragm casing is preferably so chosen that, incombination with the compliance of the air in the space between the backofthe diaphragm and the back plate of its cas- Vlili Ain the front wallof the diaphragm casing and the compliance of the volume of air in thespace between this front wall and the front surface of the diaphragm arethen selected to resonate at the high frequency end, This has the effectof removing the excessive peak otherwise obtained.

(g) The values of the acoustical resistance behind the one or moreopenings in the front plate of the diaphragm casing, the acousticalresistance behind the aforementioned tubular member, the acousticalresistance behind the one or more open- .ings in the back plate of thediaphragm casing, and the acoustical resistance behind the chambercovered opening or openings in the back plate of the diaphragm casingare then all adjusted or selected to obtain the desired frequencyresponse, which is preferably assumed to be as at as possible.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description of one or more embodimentsthereof, when read in connection with the accompanying drawings, inwhich Figure 1 shows a central sectional view of one form of myinvention,

Figure 2 is a sectional view taken on the line II-II of Figure 1,

Figure 3 is an enlarged, detail, sectional view of a portion of thecoupling member between the diaphragm and the armature,

Figure 4 is a fragmentary sectional view taken on the line IV--IV ofFig. 2,

Figure 5 is an electrical wiring diagram showing the electrical analogueequivalent to the acoustical system of the structure shown in Figures 1and 2, and

Figures 6 to 9, inclusive, are response curves showing how the outputcharacteristics vary by proper selection of the values of mass,compliance and resistance as discussed above.

Referring more particularly to the drawings, wherein similar referencecharacters designate corresponding parts throughout, there is shown, inFig, 1, a, diaphragm I within a casing 3 having a front plate 5 and aback plate 1, the diaphragm being suitably clamped at its peripherywithin the casing 3 in spaced relation to the walls 5 and 1 thereofwhereby to provide a front air space 9 between the front casing wall 5and the front surface of the diaphragm I and a back space II between theback wall 1 and the rear surface of the diaphragm I. The front wall 5 ofthe diaphragm casing is provided with one or more openings I 3 forestablishing communication between the air space 9 and the atmosphere,and the back wall 1 is provided with a plurality of openings I5 whichare disposed symmetrically about the diaphragm axis and serve toestablish communication between the back air space II and the ex` teriorof the casing 3. In the particular modification disclosed, only oneopening I3 is shown in the front plate 5 and the three openings I5 areshown in the back plate 1, but it will be obvious that any desirednumber of openings may be employed in either of the plates 5 or 1 forestablishing communication between the interior and the exterior of thecasing 3 on both sides of the diaphragm I. If desired, there may also beprovided a perforated disk or grid I1 behind the front plate 5 toprotect the diaphragm I from accidental injury. The openings I3 and I5are all covered by a layer of foraminated material I9, such as silk,constituting an acoustic resistance to the passage of sound wavesthrough the openings I3 and I5, the silk layer associated with theopening I3' being shown immediately behind the grid I1 in the particularmodification shown in the drawings.

The back plate 1 is formed with an internally threaded, hollow extension23 with which another opening 25 through the back plate 1 communicates,a plug 21 being threaded into the extenslor- 23 to form a cavity orchamber 29 behind the opening 25, and a layer of silk I9 being alsopi--Iced behind the opening 25 similar to the bpenings I5. A coil spring3l in the chamber 29 serves `to hold the associated silk layer I9against the opening 25 when the plug 21 is screwed into position.

The casing 3 is preferably molded of a suitable plastic material and hassecured to its back plate 1 a reversible electro-mechanical unit 33 forcooperation with the diaphragm i. The unit 33 includes an operating coil35 between a pair of pole pieces 31 to which flux is supplied by a pairof magnets 38, one of the pole pieces being preferably molded integrallywith the back. plate 1, and the other being molded integrally with asupporting member 39 which may be of the same material as the casing 3,the pole faces ci the polepieces 31 being suitably ground after themolding operation. Thereafter, the supporting :member 39 is mounted onthe back plate 1 by means of a plurality of screws or the like 4I, apair of washers 43' being employed on either side of an armature 45 toaccurately center 'the armature in the air-gaps between the pole pieces31.

The armature 45 constitutes the active element of the electro-mechanicaldevice 33 and is coupled to the diaphragm I by a pair of light,telescopic members 41 and 49, the formezI of which is provided wthanenlarged head 41u and the latter of which has an enlarged head 49a, asclearly shown in Fig. 3. The diaphragm i is provided with an opening Iaof a diameter sub stantially equal to the internal diameter of themember 41J and the member 49 fits through the opening la and is slidablyreceived within the tubular member 41, the area of the diaphragmimmediately surrounding the opening Ict being clamped between the heads41a and 49a. Frei erabiy, the pin or central member 49 is coated with asuitable cement prior to insertion into the tube 41, and its head may beheated with a soldering iron to cause the cement to bond the members 41and 49 firmly to each other and to the diaphragm I. The coupling tube 41and thc pin 49 are preferably made of aluminum, for lightness, and alsopreferably pass through a rubber seal '50 on the back plate 1,

By forming the reversible unit 33 and the coupling member 41-49 betweenits armature 45 and the diaphragm I in the manner above de scribed,there is provided, first, a vibiatory system constituted by the armature45, the coupling member 41-49, and the diaphragm I which is of minimummass, a condition essential to the high efficiency of my improvedtranslating device. Furthermore, mounting the pole pieces on the backplate 1 and the supporting member 39 and thereafter assembling them inthe manner heretofore described after grinding the two pole faces ofeach of the pole pieces 31 along a common plane not only insures an airgap of desired width, but avoids setting up strains in the armaturewhich, 'in conventional telephone units, causes an initial set to takeplace in the armature with a tendency to throw it initially oil centerand thus introduce distortion. Also, the use of a synthetic resin orother plastic material for the case 3 and the supporting member 39 inthe manner described eliminates the proximity of metal parts near thevarying flux, and thus eddy current losses are eliminated, whilegrinding of the pole piece faces after they have been molded in placeassures proper lining up of the pole faces, so that, with the aid of thewashers 43, accurate adjustment of the air gaps can be maintained.

Secured to an annular nange 3a on the back plate 1, as by means of aclamping ring I, is a housing 53 in which the translating device 33 andthe extension 23 andv its associated plug 21 are all enclosed. Thehousing 53 is preferably provided with an opening 55 in the wallthereof, a tubular member'51 being fitted into the opening 55 andextending radially inwardly of the housing l53. As in the case of theopenings I3, I5 and 25, a layer of silk or the like I9 may be placedover the inner end of the tubular member 51.

Figure 5 shows an electrical wiring diagram corresponding tothe'acoustical system heretofore described. If, in the acousticalsystem, it is assumed that Cris the compliance of the air in the housing53,

Cz is the compliance of the air in the back space C3 is the compliancevof the air in the resonator chamber 29y C4 is the compliance of the airin the space 9,

C5 is the compliance of the whole moving or vibratile system consistingof the diaphragm I, the armature 45 and the couplingtherebetween I1-49,

M1 is the mass of air in the tube 51,

M2 is the mass of air in the openings I5,

M3 is the mass of air in the opening 25,

M4 is the mass of air in the opening I3,

M5 is the mass of the entire vibratile system,

R1 is the' acoustic resistance provided by the silk screen at the end ofthe tube I9, v

R2 is the acoustic resistance offered by the silk screen behind theopenings I5,

R3 is the acoustic resistance offered by the silk screen behind theopening 25, and

R4 is the acoustic resistance offered by the screen behind the openingI3 (or the grid I'I) as the case may be,

then the relation of the system will be as shown in the electricalanalogue of Fig. 5.

It will be obvious,` from an inspection of Fig. 5, that the responsecharacteristic-of the system can be altered by changing the'vaiues ofthe various inductances, resistances, or capacitances. I have foundthat, for a speech microphone, the following general constants aredesirable:

(a) By proper selection of the length and internal cross-section, ordiameter, of the tube 51, the mass of air therein, as represented by theinductance M1 in Fig. 5, can be made to resonate with .the compliance C1of the air inthe housing 53 at the lowest frequency at whichtransmission is desired. In one particular microphone, this frequency isapproximately 200 cycles. It has been found that this cut-ofi frequencydetracts very little from the intelligibility of speech and greatlyreduces the effect of wind and low frequency noise which may be present.1

(b) The resonance of the mass M5 of the whole vibratile system with thecapacitance C5 of this system should be made as high in frequency aswill allow the desired low frequency response.

(c) The compliance C2 of the air in the space II is made so small that,in combination with the mass and compliance of the vibratile system, M5and C5, respectively, resonance will be obtained at the highestfrequency which it is 'desired to transmit.

(d) The value of the capacitance C3 of the air in the resonator chamber29 is preferably so chosen that, in combination with the compliance C2of the air in the space II, the compliance C5 of the vibratile system,and the mass zol M5 of the vibratile system, resonance will occur at afrequency which is about midway between the highest and lowestfrequencies which it is desired to transmit by the unit. Y

(e) The mass M3 of air in the opening 25 and the compliance C3 of theair in the resonator v chamber 29 are also made to resonate at thehighest frequency which it is desired to transmit.

(f) The mass M4 of air in the opening I3 and the compliance of the airin the space 9 are then made to resonate at the high frequency end toremove the excessive peak which would otherwise be attained.

(g) The values of the acoustic resistances R1,

R2, Rs and R4 are then adjusted to obtain the desired frequencyresponse, which, in the particular case illustrated, is assumed to be asflat as possible.

Fig. 6 shows a response curve for the system without the resistances R1,R2 and R3, the various peaks at the frequencies f1, f2, f3 and f4resulting from the various portions of the system designated by thecorresponding masses, compliances and resistances appended to theindividual peaks. By adding the resistances R1, R2 and R3 of appropriatevalue, the response curve of Fig. 7 will result. It will be noted thatthe output of the system is then represented by a much flatter responsecurve throughout the range up to frequenoy f3, the peak at frequency f4still remaining. However, the peak at the frequency f4 can be reducedconsiderably (as in Fig. 8) by properly selecting the values of M4 andC4, that is, by making the opening I3 and the air space 3 of properdimensions, and the response curve may-be made substantially flat,nally, by proper selection of vthe values of M2 and Rz to provide aresponse such as shown in Fig. 9.

In practice, I have found that it is very desirable to have the openingsIii formed in the back plate 'I behind the central portion of thediaphragm i, which executes a nearly piston-like motion. Furthermore,asv pointed out heretofore, the openings iii should preferably beuniformly and symmetrically spaced around 'the Vcenter of the diaphragm.Ir the openings I5 are not placed symmetrically with respect to thecenter of the diaphragm, or if they are formed-in the plate 1 behind aregion adjacent the periphery of the diaphragm, it has been found thatthe diaphragm has a tendency to rock which, in turn, produces rathererratic response curves. For the foregoing reasons, therefore, it isessential that the openings i5 be properly located.

Although I have shown and described but a y single embodiment of myinvention, it will be relation to each of said walls whereby to providefront and back air spaces, respectively, in front of and behind saiddiaphragm, signal translating means exterior to saidcasing and includingan active element, means coupling said element to said diaphragm formovement in unison, and a housing coupled to said back casing Wall andenclosing said translating means, said front casing wall having at leastone opening extending therethrough and aiording communication betweensaid front space and the atmosphere, said back casing Wall having atleast one opening extending therethrough and affording communicationbetween said back space and the interior of said housing, and saidhousing having at least one opening extending through a Wall thereof andaffording communication between the interior of said housing and theatmosphere.

2. The invention set forth in claim A1 characterized in that each ofsaid openings is covered by a foraminous material constituting anacoustic resistance to the passage of sound waves therethrough.

3. The invention set forth in claim 1 characterized by the addition ofan open ended tubular member extending inwardly of said housing throughthe opening therein whereby communication between the interior of saidhousing and the atmosphere is through said tubular member.

4; The invention set forth in claim 1 characterized by the addition ofan open ended tubular member extending inwardly of said housing throughthe opening therein whereby communication between the interior of saidhousing and the atmosphere is through said tubular member, andcharacterized further in that said tubular member is of such length andinternal crosssectional area that the mass of air therein resonates withthe compliance of the air in said housing at substantially apredetermined low frequency such that said low frequency will besubstantially the lowest frequency that said apparatus will translate.

5. The invention set forth in claim l characterized by the addition ofan open ended tubular member extending inwardly of said housing throughthe opening therein whereby communication between the interior of saidhousing and the 4atmosphere is through said tubular member, andcharacterized further in that the openings in both said casing walls andthe inner end oi said tubular member are all covered by a foraminousmaterial constituting an acoustical resistance to the passage of soundwaves therethrough.

6. In signal translating apparatus, the combination of a casing having afront wall and a back `least one opening extending therethrough andaiording communication between said iront space and the atmosphere, saidback casing wall having at least two openings extending therethrough, atleast one of said last named openings affording communication betweensaid back space and the exterior of said casing, means on said back wallbehind the other of said openings therein providing a closed chambertherebehind, said last named opening affording communication betweensaid chamber and said back space, and a housing coupled to said backplate and enclosing said translating means and said chamber providingmeans, said housing also having an opening extending through a wallthereof and ailording communication between the interior oi said housingand the atmosphere.

'i'. The invention set forth in claim 6 charactcrized by the addition ofa tubular member extending through said housing'wall opening into theinterior of said housing whereby communication between the interior ofSaid housing and the atmosphere is through said tubular member.

Ei, The invention set forth in claim f5 characterized by the addition ofa tubular member extending through said housing wall opening into theinterior oi said housing whereby communication between the interior ofsaid housing and thc atmosphere is through said tubular member, andcharacterized still further by the addition of a cover of foraminousmaterial over all of the openings in each of said casing plates and overthe inner end of said tubular member, said ioraminous coversconstituting acoustical resistances to the passage of sound wavesthrough said openings and through said tubular member, and the values ofsaid acoustical resistances being such as to give a substantially atresponse characteristic to said apparatus.

LESLIE J. ANDERSON.

